Mechanistic issues in BOLD based fMRI were further examined by comparing BOLD images with perfusion change measurements, demonstrating that in gray matter areas in the visual cortex, absolute and relative CBF changes in humans during photic stimulation were 31 1 11 SD ml/100 g tissue/min and 43 1 16 SD% (n = 12), respectively, while the relative oxygen consumption change was close to zero. These findings were in agreement with previous results using positron emission tomography.